R. Arad

Observation of faster-than-diffusion magnetic field penetration into a plasma

Spatially and temporally resolved spectroscopic measurements of the magnetic field, electron density, and turbulent electric fields are used to study the interaction between a pulsed magnetic field and a plasma. In the configuration studied (known as a plasma opening switch) a 150 kA current of 400 ns-duration is conducted through a plasma that fills the region between two planar electrodes. The time-dependent magnetic field, determined from Zeeman splitting, is mapped in three dimensions, showing that the magnetic field propagation is faster than expected from diffusion based on the Spitzer resistivity. Moreover, the measured magnetic field profile and the amplitude of turbulent electric fields indicate that the fast penetration of the magnetic field cannot be explained by an anomalously high resistivity. On the other hand, the magnetic field is found to penetrate into the plasma at a velocity that is independent of the current-generator polarity, contradictory to the predictions of the Hall-field theory...

Spectroscopic investigations of a dielectric-surface-discharge plasma source

Spectroscopic investigations of the properties of a plasma produced by a flashboard plasma source, commonly used in pulsed plasma experiments, are presented. The plasma is used to prefill a planar 0.4 μs conduction time plasma opening switch (POS). A novel gas-doping technique and a secondary surface flashover plasma source are used to locally dope the plasma with gaseous and solid materials, respectively, allowing for spatially resolved measurements. The electron density, temperature, and plasma composition are determined from spectral line intensities and line profiles. Detailed collisional-radiative modeling is used to analyze the observed line intensities. The propagation velocity and divergence angle of various ions are determined from time-of-flight measurements and Doppler broadening of spectral lines, respectively. This allows for distinguishing the secondary plasma ejected from the POS electrodes from the plasma of the flashboard source.

Spectroscopic investigation of fast (ns) magnetic field penetration in a plasma

The time‐dependent magnetic field spatial distribution in a coaxial positive‐polarity plasma opening switch (POS) carrying a current ≂135 kA during ≂100 ns, was investigated by two methods. In the first, ionic line emission was observed simultaneously for two polarizations to yield the Doppler and Zeeman contributions to the line profiles. In the second method, the axial velocity distribution of ions was determined, giving the magnetic field through the ion equation of motion. This method requires knowledge of the electron density, here obtained from the observed particle ionization times. To this end, a lower bound for the electron kinetic energy was determined using various line intensities and time‐dependent collisional‐radiative calculations. An important necessity for POS studies is the locality of all measurements in r, z, and θ. This was achieved by using laser evaporation to seed the plasma nonperturbingly with the species desired for the various measurements. The Zeeman splitting and the ion moti...

Spectroscopic investigations of the plasma behavior in a plasma opening switch experiment

The electron density, the electron kinetic energy, the particle motion, and electric fields in a coaxial positive‐polarity plasma opening switch (POS) were studied using spectroscopic diagnostics. A gaseous source that injects the plasma radially outward from inside the inner POS electrode was developed. The plasma was locally seeded with various species, desired for the various measurements allowing for axial, radial, and azimuthal resolutions both prior to and during the 180 ns long current pulse. The electron density was determined from particle ionization times and the electron energy from line intensities and time dependent collisional‐radiative calculations. Fluctuating electric fields were studied from Stark broadening. The ion velocity distributions were obtained from emission‐line Doppler broadenings and shifts. The early ion motion, the relatively low ion velocities and the nearly linear velocity dependence on the ion charge‐to‐mass ratio, leads to the conclusion that the magnetic field penetrat...

Investigation of the ion dynamics in a multispecies plasma under pulsed magnetic fields

The interaction between a moving magnetic-field front and a low-collisionality plasma consisting of different ion species is investigated using spatially and temporally resolved spectroscopic techniques. The experiment is carried out in a plasma-opening-switch configuration, in which a current rising to 150 kA in 400 ns is conducted through a plasma that prefills the region between two planar electrodes. Ion-species separation is found to occur, similarly to the results reported for a 80 ns duration plasma-opening-switch experiment of cylindrical geometry, which was not necessarily expected since in the present experiment plasma pushing is more substantial. The separation, in which the light-ion plasma (protons) is reflected while the heavy-ion plasma (carbon) is penetrated by the propagating magnetic-field, is investigated by determining the electron density from the temporal evolution of spectral lines, the nonprotonic ion velocities from line-emission Doppler shifts, and the proton velocity distributio...

Plasma dynamics in pulsed strong magnetic fields

Recent investigations of the interaction of fast-rising magnetic fields with multi-species plasmas at densities of 1013–1015 cm−3 are described. The configurations studied are planar or coaxial interelectrode gaps pre-filled with plasmas, known as plasma opening switches. The diagnostics are based on time-dependent, spatially resolved spectroscopic observations. Three-dimensional spatial resolution is obtained by plasma-doping techniques. The measurements include the propagating magnetic field structure, ion velocity distributions, electric field strengths, and non-Maxwellian electron energy distribution across the magnetic field front. It is found that the magnetic field propagation velocity is faster than expected from diffusion. The magnetic field evolution cannot be explained by the available theoretical treatments based on the Hall field (that could, in principle, explain the fast field propagation). Moreover, detailed observations reveal that magnetic field penetration and plasma reflection occur si...

Ion dynamics in a two-ion-species plasma

The ion dynamics in a two-ion-species plasma pushed by a magnetic field pressure is examined. It is shown that the behavior of such a plasma may significantly differ from the behavior of a single-ion-species plasma, in which the current is carried by the electrons and the ions move in the direction of the magnetic field force. Cases are demonstrated in which the current is carried by the ions, the ions acquire a velocity with a large component perpendicular to the magnetic field force, and one of the two ion species does not move in the direction of the magnetic field force exerted on the plasma. Experimental evidence for such an ion motion perpendicular to the magnetic field force is described.  2002 Elsevier Science B.V. All rights reserved.

Novel gas-doping technique for local spectroscopic measurements in pulsed-power systems

A novel method for doping plasmas in pulsed-power experiments with gaseous elements has been developed. A fast gas valve, a nozzle, and a skimmer are used to generate an ultrasonic gas beam that is injected into a planar-geometry microsecond plasma-opening switch (POS). An array of ionization probes with relatively high spatial and temporal resolutions was developed for diagnosing the absolute injected-gas density and its spatial profile. The properties of the gas column were also studied using spectroscopy of line emission that results from the interaction of the doped gas with the POS prefilled plasma. The doped column is found to have a width of ≈1 cm and a density of (0.8–1.7)×1014 cm−3. Observations of characteristic emission lines from the doped atoms and their ions allow for various spectroscopic measurements, such as the magnetic field from Zeeman splitting and the ion velocity distributions from Doppler shifts, that are local in three dimensions. It is shown that this gas doping technique can als...

Magnetic field propagation in a two ion species planar plasma opening switch

Three fluid plasma evolution equations are applied to the problem of magnetic field propagation in a planar plasma opening switch. For certain initial conditions in which Hall parameter H∼1, magnetic field penetration due to the Hall field, initially, as expected, either opposes or adds to the hydromagnetic pushing, depending on the polarity of the magnetic field relative to the density gradient. Later, however, the plasma pushing by the magnetic field is found in the case studied here to modify the plasma density in a way that the density gradient tends to align with the magnetic field gradient, effectively turning off the Hall effect. The penetration of the magnetic field then ceases and plasma pushing becomes the dominant process.

Visible-light spectroscopy of pulsed-power plasmas (invited)

We describe the investigations of the plasma behavior in three pulsed‐power systems: a magnetically insulated ion diode, and plasma opening switch, and a gas‐puffed Z pinch. Recently developed spectroscopic diagnostic techniques allow for measurements with relatively high spectral, temporal, and spatial resolutions. The particle velocity and density distributions within a few tens of microns from the dielectric‐anode surface are observed using laser spectroscopy. Fluctuating electric fields in the plasma are inferred from anisotropic Stark broadening. For the plasma opening switch experiment, a novel gaseous plasma source was developed which is mounted inside the high‐voltage inner conductor. The properties of this source, together with spectroscopic observations of the electron density and particle velocities of the injected plasma, are described. Emission line intensities during the switch operation are discussed. In the Z‐pinch experiment, spectral emission‐line profiles of various charge‐state ions ar...